res_pjsip: Ensure in-dialog responses have an endpoint associated.

res_pjsip: Ensure in-dialog responses have an endpoint associated.

When handling incoming messages we determine if it is associated with
a dialog. If so we use that to determine what serializer and endpoint
to use for the message. Previously this would pass the endpoint to the
endpoint lookup module to actually place the endpoint completely on the
message. For in-dialog responses, however, this did not occur as
dialog processing took over and the endpoint lookup did not occur.

This change just places the endpoint in the expected spot immediately
instead of relying on the endpoint lookup module. In-dialog responses
thus have the expected endpoint.

AST-1459 #close

Review: https://reviewboard.asterisk.org/r/4146/
........

Merged revisions 427490 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@427491 65c4cc65-6c06-0410-ace0-fbb531ad65f3